Inkjet imaging techniques have become very popular in commercial and consumer applications. The ability to use a personal computer and desktop printer to print a color image on paper or other receiving substrate has extended from dye-based inks to pigment-based inks. The latter provide brilliant colors and more durable images because pigment particles are more durable than dyes on the receiving substrate.
Thermal inkjet print heads are commercially available from Hewlett Packard Corporation or LexMark Corporation in inkjet printers commercially available from Hewlett Packard Corporation, Encad Inc., Mimaki Corporation, and others.
Piezo inkjet print heads are commercially available from Topaz Technologies (Sunnyvale, Calif.), Epson Corporation (Torrance, Calif.). Data Products (Woodland Hills, Calif.), Modular Ink Technologies (Dallas, Tex.). and others. These printheads differ in physical properties such as frequency and drop volume and the inks to be used in them often require different physical properties such as viscosity. Such print heads are used in piezo inkjet printers commercially available from Idanit Technologies, Ltd. of Rishon Le Zion Israel; Raster Graphics of San Jose, California; Vutek Inc. of Meredith, N.H.; Olympus Optical Co. Ltd. of Tokyo, Japan and others.
Inkjet printers have been in general use for wide-format electronic printing for applications such as, engineering and architectural drawings. Because of the simplicity of operation, economy of inkjet printers, and improvements in ink technology the inkjet imaging process holds a superior growth potential promise for the printing industry to produce wide format, image on demand, presentation quality durable graphics.
The components of an inkjet system used for making image graphics can be grouped into three major categories:
1 Computer, software, printer with print head and plumbing components. PA1 2 Ink. PA1 3 Receiving substrate.
The computer, software, and printer will control the size, number and placement of the ink droplets and will transport the receiving substrate. The ink will contain the colorant or pigments which form the image and the receiving substrate provides the medium which accepts and holds the ink. The quality of the inkjet image graphic is a function of the total system. However, the composition and interaction between the ink and receiving substrate is important in an inkjet system only after the ink has traversed the plumbing of the printer and been jetted through the print head without damage to either the print head or the ink.
Image quality is what the viewing public and paying customers will want and demand to see. Many other demands are also placed on the inkjet media/ink system from the print shop, such as rapid drying, humidity insensitivity, extended shelf life, waterfastness and overall handleability. Also, exposure to the environment can place additional demands on the media and ink (depending on the application of the graphic). Typically, outdoor durability includes withstanding temperature extremes, exposure to moisture ranging from dew to rainstorms, and colorfast stability in the face of sunlight's ultraviolet light.
The chemical formulation of the pigmented inkjet ink has considerable complexity due to the requirement of continued dispersion of the pigment particles in the ink supply vessel and during jetting of the ink, yet rapid drying once the ink has been jetted on to the receiving substrate.
Piezo inkjet inks have distinct advantages over thermal inkjet inks. Thermal inkjet inks are limited to water-based systems. Because of the high temperatures involved in the thermal print head jetting process, the use of binders are limited because such binders form a residue on the heating elements of the print heads, a problem that is called "kogation" in the art. The presence of residue results in failure of the print heads to provide quality image graphics or even their need to be replaced on a regular basis.
Further, in order to prevent these water-based thermal inkjet systems from drying too early, i.e., while in the print head nozzles, these inks contain a high concentration of glycols or other type of humectant. This high concentration of glycols prevents the inks from drying completely once the thermal inkjet ink is jetted to form the image on the receiving substrate. Because as noted above, these inks have essentially no binder in their formulation, which would otherwise have permitted the formation of a film of the ink drops on the receiving substrate after the evaporation of water.
The combination of these jetting and drying conditions for thermal inkjet inks has forced thermal inkjet inks to be jetted onto receiving substrates that have a specialized ink-receptive layer. Several examples of specialized inkjet media include U.S. Pat. Nos. 5,747,148; 5,389,723; and 5,472,789. All of these inkjet media concern the issue of the disposition of liquid after jetting has occurred for pigment-based inkjet inks.
Moreover, because thermal inkjet inks cannot have a binder in their formulation, durability from exposure to the effects of the environment for the resulting image graphic is obtained by the use of an overlaminate, typically provided by a clear coating of a rapid-drying solution or a film that adhesively laminates to the resulting image.
Piezo ink print heads jet their ink by a mechanical action as opposed to heating. Piezo inks can be aqueous or non-aqueous and can include a film-forming binder. The viscosities of these inks are higher than those of thermal inkjet inks. Thermal inkjet inks typically have viscosities around about 3-5 centipoise (mPa.multidot.sec), whereas pigmented piezo inks typically have a viscosity of from about 10 to about 20 mPa.multidot.sec at 25.degree. C. depending on the type of piezo jet print head and the particulars of its specifications and performance.
However, viscosities of piezo inks are orders of magnitude lower than screen print inks used in the printing industry. Screen print inks typically have viscosities of at least about 1000 mPa.multidot.sec at time of usage. In commercially available screen print inks from Minnesota Mining and Manufacturing Company, such inks include pigment, binder, plasticizer, stabilizers, silicone oils, and solvents. The percent solids of such inks are typically between about 30 and 40 percent. In one of such inks, a fluorochemical flow agent is added for assuring complete covering of the image graphic as a clear coating. In other of such inks, to assure porosity and facilitate air passage, a minor amount of silica particle is dispersed. But none of the inks has both a fluorochemical flow agent and dispersed silica particles, because the properties of one would counteract the properties of the other.
Piezo ink jet inks have their own set of problems to overcome to achieve a high quality image with acceptable durability for its intended usage. A variety of others have attempted to develop and market piezo ink jet inks. Among these that use volatile organic solvent systems for delivery of ink through the print head and to the receiving substrate are Salsa brand inks marketed in conjunction with Salsa brand printers from Signtech of San Antonio, Tex.; Crisper brand inks from Precision Color of Newbury, Berkshire, U.K., and piezo inks from Inkware Corporation of Las Vegas, Nev., USA.
However advanced these other piezo ink jet inks may be, there are complications attendant to how an organic solvent-based piezo ink jet can successfully be jetted through piezo ink jet print heads having drop sizes most often ranging from about 30 to about 70 picoliters (pL) that are common for the print heads used in wide format piezo ink jet printers commercially available from the printer manufacturers identified above. Most particularly the quality and durability of the resulting image graphic is directly dependent on the ink formulation that navigates the jetting process yet forms an image graphic on the receiving substrate suitable for intended usages.